This invention relates generally to apparatus utilized to connect optical fibers, and more particularly to such apparatus which include expanded beam terminations.
One of the major problems associated with the connection of optical fibers arises from the need to align their respective cores with great precision. The absolute alignment accuracy necessarily depends not only upon the core diameter and the particular type of fiber, but also upon the maximum permitted joint attenuation. One method of minimizing attenuation utilizes fiber splices, which are permanent joints in the optical fibers somewhat analogous to soldered joints in an electrical system. A number of techniques for joining optical fibers, including fusion splices, V-groove connections, and sleeve splices have been used in the past. Sleeve splices have obvious mechanical advantages for single fiber field splices, but may have the disadvantage of "slop" due to the clearance required to insert the fibers in the sleeve. Fusion splices are also attractive, but they require precise external alignment of the fibers to be joined. V-groove techniques, on the other hand, provide relatively accurate alignment with low attenuation, and are adaptable to connecting multiple pairs of optical fibers. However, connection methods which utilize V-grooves are often unnecessarily complicated by their requirement for complex splicing jigs and associated equipment. It would, therefore, be desirable to provide an optical fiber connector which minimizes attenuation as in the case of fiber splices, but which facilitates installation in the field.
Demountable connections for optical fibers, which are equivalent to electrical plugs and sockets, have also been used to provide such field installability. One prior art demountable optical connector utilizes a butt joint termination of the fibers which are usually protected by a metal or ceramic ferrule accurately locating the fiber ends. In order to terminate an optical fiber in such ferrules, the fiber's coating is removed over a short length from its end, with the fiber being fed into the ferrule filled with a suitable adhesive. The fiber is then polished back flush with the ferrule end. One drawback to the use of butt joint terminations is that they are typically manufactured at the factory in order to reduce the possibilities of introducing contaminants such as moisture and dust, and to permit precise control of geometry.
A second major method of demountable optical connection uses the well-known expanded beam technique for joining fibers. In this approach, the size of the transmitted beam is increased by one half of a connector, the expanded beam being reduced again to a size compatible with the core of the receiving fiber by the second half of the connector. Such expansion can be achieved by using lenses, including glass, plastic, or graded index lenses.
In a demountable connector, an important contribution to the loss is lateral misalignment, since a number of machining tolerances are usually involved. Axial separation of fiber ends also induces a loss which is dependent upon the numerical aperture of the fiber. In a well designed connector or a splice, this separation should be small and its effects minimized through use of an index matching medium. However, while index matching is normally used in a splice, its use in a demountable butt joint connector is a problem owing to the collection of dust and dirt. Angular misalignment of the fiber cores also contributes to connector loss since some of the light incident at the receiving fiber core is not within its acceptance angle. Another contribution to connection loss is Fresnel reflection at the interfaces of the fibers.
In expanded beam terminations, when a prepared fiber end is fixed at the focus of a lens, a collimated beam with a diameter greater than the fiber core diameter emerges from the lens. An optical connector is produced when two such terminatins are aligned. The required connection tolerances are reduced since the increased beam diameter allows greater lateral misalignment of the expanded beam terminations then directly butting fibers. Moreover, owing to the collimation of the beam, a small separation of the terminations can be tolerated without significantly increasing the attenuation. The increased beam diameter also reduces the effect of dust on the connector attenuation since the separation of the terminations minimizes the risk of permanent damage arising from grit, scratching, or chipping the optical surfaces when the connector is inadvertently coupled in a dirty condition. These conditions make this an important termination technique for rugged, field installable connectors.
One prior art approach utilizing the expanded beam technique is described in U.S. Pat. No. 4,421,383--Carlsen. Light from an optical fiber is coupled to an integral optical-quality plastic connector body having an annular planar reference surface substantially perpendicular to the optical axis. A convex lens surface is molded, recessed inward from the reference surface, while the opposite axial end of the body has a central cylindrical cavity within which the fiber is held one focal length from the lens surface by a molded elastomeric fiber holder. Two such expanded beam terminations are held together by a slightly elastic cylindrical tube having a shaped lip which snap fits over raised rings formed about the outer surface of the connector bodies.
One drawback to the above claimed invention is that it incorporates only spherical, convex lens surfaces. That is, the convex lens used in the connector of U.S. Pat. No. 4,421,383 is generally incapable of reducing aberrations as in the case of aspheric lens surfaces which are altered slightly from the typical spherical surface encountered in such convex lenses. Such aspheric surfaces, especially when integrated within molded bodies such as that disclosed in U.S. Pat. No. 4,421,383, are often difficult to produce and generally cost-ineffective. Furthermore, plastics suitable for optical applications are available in a limited refractive index and dispersion range only. Many plastics scratch easily and are prone to the development of yellowing, haze, and birefringence. The use of abrasion-resistant and anti-reflective coatings, as disclosed in U.S. Pat. No. 4,421,383, has not fully solved those failings. Moreover, plastic optical elements are subject to distortion from mechanical forces, humidity, and heat. Both the volume and refractive index of plastics vary substantially with changes in temperature, thereby limiting the temperature interval over which they are useful.
The overall properties of glass render it generally superior to plastic as an optical material. Glass is a much better substrate for the application of multi-layer anti-reflection coatings because it is chemically inert, dimensionally stable, and can be coated at elevated temperatures. As described above, glass also has excellent performance over a broad range of temperature, humidity, and other environmental conditions. This performance is due to its low coefficient of thermal expansion, its essential imperviousness to water absorption, its high resistance to other environmental attacks (e.g., salt spray, fungus, and acids), and its resistance to other atmospheric contaminants. Additionally, glass has a very high mechanical strength, allowing precision optical elements formed of glass to perform without optical degradation, or mechanical deformation while under stress.
Precision optical elements of glass are customarily produced via one of two complex, multi-step processes. In the first, a glass batch is melted in a conventional manner and the melt formed into a glass body having a controlled and homogenous refractive index. Thereafter, the body may be reformed utilizing well-known repressing techniques to yield a shape approximating the desired final article. The surface figure and finish of body at this stage of production, however, are not adequate for image forming optics. As a result, the rough article is fine annealed to develop the proper refractive index, while the surface figure is improved via conventional grinding practices. In the second method, the glass melt is formed into a bulk body which is immediately fine annealed and substantially cut and ground to articles of a desired configuration.
Both processes are subject to similar limitations. The surface profiles that are produced through grinding are normally restricted to conic sections, such as flats, spheres, and parabolas. Other shapes and, in particular general aspheric surfaces are difficult to grind. In both processes, the ground optical surfaces are polished employing conventional, but complicated, polishing techniques which strive to improve surface finish without comprising the surface figure. In the case of aspheric surfaces, this polishing demands highly skilled and expensive hand working. A final finishing operation, viz., edging, is commonly required. Edging insures that the optical and mechanical axes of a spherical lens coincide. However, edging does not improve the relationship of misaligned aspheric surfaces, which factor accounts in part for the difficulty experienced in grinding such lenses. It would, therefore, be desirable to design an expanded beam waveguide connector which employs aspherical lenses to reduce aberrations, the lens being manufactured of optical quality glass rather than plastic, and being produced through direct molding to its finished state, thus eliminating the grinding, polishing, and edging operations which are especially difficult and time consuming for aspherical lenses.
One such prior art approach utilizing aspheric lenses to connect expanded beam terminations includes a molded glass aspheric element having active imaging surfaces and percision flats on both the front and rear. The flat adjacent to the aspheric surface serves as a reference surface such that, when opposing lenses are mounted within the connector element having precision stainless steel ball spacers between them, they are true to each other to within about 15 seconds of arc. Compression rings mounted behind the lenses provide elastic opposing forces needed to keep the lenses in positive contact with the ball spacers.
Since the contact length of the lens cell diameter within its housing is very small, it provides the lens with the freedom to align angularly while controlling lateral displacement. Lateral displacement is also provided by a cell molded around the lens. In order to connect the lens cell with its respective optical fiber, the fiber is inserted within and secured to a ferrule with the fiber/ferrule assembly being correctly positioned on the lens through use of a complex fiber positioning advice comprised of a quad cell detector, preamp, feedback electronics and motor drive.
Due to the relatively simple configuration of the lens element, a connector manufactured in accordance with the above described approach may be accomplished through molding. However, the manufacturing of such elements requires a two-step process of molding the lens followed by molding the lens cell thereafter. Furthermore, the glass composition utilized in such applications necessitates production temperatures in excess of 500.degree. C., and is substantially incapable of being molded into complex and concave surfaces. Other drawbacks, such as the relative complexity of the equipment required to position the fiber relative to the optical axis of the aspheric lens, and the precision machining necessitated by the compression ring and ball spacers, also complicate the production of such prior art devices. It would, therefore, be desirable to provide a precision optical element for use in expanded beam waveguide connectors which is easy and cost-effective to fabricate, and readily installed in the field.